Referring to FIG. 1, a wireless communication system 100 includes a plurality of Base Stations (BSs) 110a, 110b and 110c and a plurality of User Equipments (UEs) 120a to 120i. The wireless communication system 100 may include homogeneous networks or heterogeneous networks. Heterogeneous networks refer to networks in which different network entities coexist, such as a WiFi, IEEE 802.16, LTE, etc. A BS is usually a fixed station that communicates with UEs. Each BS 110a, 110b or 110c provides services to its specific geographical area 102a, 102b or 102c. For the purpose of improving system performance, the specific service area of the BS may further be divided into a plurality of smaller areas 104a, 104b and 104c. These smaller areas may be called cells, sectors or segments. In an Institute of Electrical and Electronics Engineers (IEEE) 802.16 system, cell Identities (IDs) (Cell_IDs or IDCells) are assigned from the perspective of a whole system, whereas sector IDs or segment IDs are assigned from the perspective of the service coverage of each BS, ranging from 0 to 2. The UEs 120a to 120i, which are mobile or fixed, are generally distributed over the wireless communication system 100. Each UE may communicate with at least one BS on a downlink and an uplink at a point of time. The communication may be conducted in Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), Single Carrier-FDMA (SC-FDMA), Multi Carrier-FDMA (MC-FDMA), Orthogonal Frequency Division Multiple Access (OFDMA), or a combination of them. Herein, a downlink refers to a communication link directed from a BS to a UE and an uplink refers to a communication link directed from the UE to the BS.
In the mean time, various standardization projects for construction of a wireless communication system have been progressed, and the improved standard developed from the existing standard is being settled. Generally, systems based on the improved standard are required to provide backward compatibility with a user equipment and a base station, which are implemented based on the previous standard. Accordingly, a method for providing backward compatibility from an improved system to an existing user equipment or an existing base station will be required.